A magnetic resonance imaging apparatus is an imaging apparatus which excites nuclear spin of an object placed in a static magnetic field with an RF (Radio Frequency) signal having the Larmor frequency and reconstructs an image based on magnetic resonance signals emitted from the object due to the excitation.
In imaging with the use of a magnetic resonance imaging apparatus, radio frequency (RF) signals are applied to an object in order to obtain magnetic resonance signals. Application of radio frequency signals heats up an object and increases body temperature of the object. For the above reason, SAR (Specific Absorption Ratio) is defined as energy absorbed per unit mass of an object from the perspective of safety. In addition, upper limit values of SAR, i.e. safety reference values of SAR are defined in IEC (International Electrotechnical Commission) standards (IEC60601-2-33).
More specifically, SAR (its unit is W/kg) is defined as energy of RF signal(s) absorbed by one kilogram of biological tissue. An upper limit value of time average SAR in arbitrary ten seconds and an upper limit value of time average SAR in the last six minutes are also defined for each imaging part such as the whole body, the head part, etc. Hereinafter, the above-described time average SAR in arbitrary ten seconds is simply referred to as 10-second average SAR, and the above-described time average SAR in the last six minutes is simply referred to as 6-minute average SAR.
Meanwhile, in magnetic resonance imaging of an object, combining plural imaging sequences and consecutively executing the respective imaging sequences in the combined order is generally performed. In general, the above-described SAR is different depending on each imaging sequence. In addition, an imaging time of an individual imaging sequence may be shorter than six minutes in many cases. Therefore, the 6-minute average SAR is different depending on the execution order of imaging sequences. Japanese Patent Application Laid-open Publication No. 2011-143235 discloses technology of automatically determining execution order of imaging sequences so that the 6-minute average SAR does not exceed its upper limit value when plural imaging sequences are performed.
Recently, upper limit values of “long MR examination specific absorbed energy” are newly defined in the third edition of IEC60601-2-33 of IEC standards, in addition to the above-described 10-second average SAR and 6-minute average SAR. This is a rule that continuation of an examination is prohibited when the total amount (i.e. an accumulated value or an integrated value) of SAR of this examination exceeds the upper limit value of the total amount of SAR for one examination. Hereinafter, long MR examination specific absorbed energy is simply referred to as SAE (Specific Absorbed Energy).
When SAE of a certain examination exceeds its upper limit value, this examination cannot be continued under the above-described rule even if this examination is in progress. The possibility that SAE reaches its upper limit value is high, especially (a) when an examination requiring a long imaging time such as whole body imaging is performed and (b) when imaging is interrupted on the way and then imaging is resumed.
For the above reasons, a magnetic resonance imaging apparatus, whereby a user such as an inspection engineer or a doctor can easily recognize relationship between an SAR value and its upper limit value not only in a planning stage of an examination but also in the middle of an examination, has been desired.